High resolution encoder

ABSTRACT

A high resolution encoder having a relatively high number of magnetic pole pairs with a magnetic flux strength sufficient for use in high resolution applications. Magnetic pole pair spacing varies from about 0.01 inch to about 0.1 inch with each magnetic pole pair having a magnetic flux in the range of about 2 gauss to about 700 gauss at a distance of 0.036 inch at a temperature of +20 degrees centigrade. This high resolution encoder is especially well-suited for use with magnetically encoded targets in sensor bearings.

This is a continuation of application Ser. No. 408,592 filed 9/18/89,now U.S. Pat. No. 4,987,415.

This invention relates to encoders which provide a relatively highnumber of magnetic pole pairs with a threshold magnetic flux strengthfor use in high resolution applications. This invention is especiallywell-suited for use with magnetically encoded targets in sensor bearingscapable of providing relatively high resolution output signals, asrequired in various automotive applications.

This invention can be utilized with elongated encoders or with annularencoders or with encoders having other configurations, depending on thespecific application. For example, an elongated encoder could be used todetermine the position of the rod portion of a shock absorber. When usedin sensor bearings, the encoders of the present invention are designedto provide a one-piece unit which can be installed in a bearing havingan outer race and an inner race, or a stationary thrust plate and arotating thrust plate. This invention is suitable for use with bearingshaving a wide range of designs, including ball bearings, rollerbearings, tapered roller bearings, and needle bearings. Either thrust orradial bearing configurations may be utilized; in addition, taperedbearings are included. Antifriction bearings having rolling elements,with or without a cage or retainer or separator, are included, as wellas plain bearings having no rolling elements.

Conventional bearing encoders normally utilize a much smaller number ofNorth-South magnetic pole pairs (MPP) than the maximum possible numberof MPP. A smaller number of MPP results in a lower resolution.Techniques for maximizing the number of poles are not normally explored,since relatively low resolutions are acceptable for many automotivebearing products which incorporate an encoder. Maximizing the number ofMPP decreases the pole spacing between adjacent pole pairs, thusdecreasing the magnetic flux density emanating from each MPP. Therefore,if a particular minimum flux density is required for a specific productapplication, care must be taken to ensure that too many MPP are notplaced on an encoder so as to decrease the flux density to anunacceptably low value. The easiest way to ensure adequate flux densityis to keep the pole spacing relatively large, which results in arelatively low number of MPP. Again, this is no problem when theresolution requirements for a specific application are not stringent;however, if relatively high resolution is a requirement, then theencoder of the present invention is extremely useful.

A typical encoder for a prior art sensor bearing has a minimum polespacing of approximately 0.150 inch, which corresponds to about 40magnetic pole pairs for an annular encoder with a diameter of about twoinches. An example of of the prior art is disclosed in U.S. Pat. No.4,732,494 for a "Bearing or Roller Bearing With Data Sensor" issued onMar. 22, 1988, in the names of Roger Guers and Georges Godard.

The encoders of this invention provide a relatively high number of MPPprecisely positioned on a magnetizable member in order to satisfy highresolution application requirements. More importantly, each total numberof MPP for an encoder has an optimal magnetic flux density associatedwith each pole pair. This flux density-MPP number combination has aunique and novel range of values for specific applications, such asbearings. An equation which describes this flux relationship in terms ofpole spacing follows:

    F.sub.r =(K.sub.1)(X)-(K.sub.2)(140X.sup.2)+(K.sub.3)(7920X.sup.3)

where

K₁ =158 to 1056 gauss per inch,

K₂ =158 to 1056 gauss per inch²,

K₃ =158 to 1056 gauss per inch³,

X=Pole Spacing between adjacent MPP, and

F_(r) =Reference magnetic flux density range (in gauss) at +20 degreesCentigrade, when the numerical values of K₁, K₂, and K₃ are equal.

This equation is good for an air gap of 0.036 inch between the outersurface of the encoder and the sensitive portion of the sensor. Allmagnetic flux densities herein are measured with a Hall sensor throughthe same size air gap of 0.036 inch. The equation is also good for mostmagnetic materials, such as strontium ferrite, barium ferrite,samarium-cobalt, or neodymium-iron-boron.

An annular encoder's number of MPP can easily be determined, since

    Number of MPP=(D)(π)/X

where,

D=Diameter of annular encoder, and

X=Pole Spacing,

Briefly described, the encoder of the present invention comprises amagnetizable member having multiple magnetic pole pairs (MPP) spacedalong the member at equal intervals. The encoder has a relatively highnumber of MPP, and each of these MPP has a predetermined minimum orthreshold magnetic flux density. This is important, for example, toensure that a Hall sensor installed in the proximity of the encoder willswitch on and off through an air gap which has some maximum value for aspecific product configuration. This relatively high number of MPP, eachhaving a minimum or threshold magnetic flux density, provides relativelyhigh resolution capabilities to the product.

The method of magnetizing the encoder involves the following basicsteps:

(a) studying the specific application requirements in order to determinethe number of magnetic pole pairs (MPP) required to get the resolutiondesired and to determine which encoder configuration is most suitablefor that particular application;

(b) determining the maximum air gap associated with that specificencoder configuration for that specific application;

(c) determining the minimum or threshold magnetic flux density requiredto ensure that the sensor will switch on and off through the maximum airgap condition, with worst-case conditions for any other known variablessuch as temperature;

(d) determining which magnetic materials would suffice to manufacturethe encoder so that a sufficient number of MPP will be placed on theencoder, each MPP having a precise placement and a magnetic flux densityat least equal to the value determined in step (c) above, so that thehigh resolution requirements of the specific application are met;

(e) selecting from the group of eligible magnetic materials, determinedin step (d) above, to provide the optimal material which is most costefficient and will meet the performance requirements of that particularapplication; and

(f) magnetizing an annular member made from the optimal material,selected in step (e) above, with at least the number of MPP required forthat specific high resolution applition so that each MPP has a magneticflux density at least equal to the value determined in step (c) above.

This invention, as well as its many advantages, may be furtherunderstood by reference to the following detailed description anddrawings in which:

FIG. 1 is a perspective view of an elongated encoder of the presentinvention;

FIG. 2 is a perspective view of an annular encoder, suitable for use ina thrust bearing, of the present invention;

FIG. 3 is an axial end view of an annular encoder, showing the magneticpole pairs spaced circumferentially around an encoder used in a radialbearing;

FIG. 4 is a radial side view of the encoder in FIG. 2, illustrating themagnetic flux emanating from magnetic poles of an encoder used in athrust bearing;

FIG. 5 is an illustration of the magnetic flux density sinusoidal outputcurve resulting from the encoders shown in FIGS. 1-4; and

FIG. 6 is a graph showing the minimum pole spacing which can be obtainedfor a corresponding magnetic flux density.

Referring to the drawings and more particularly to FIGS. 1 and 2, thepreferred embodiment of this invention comprises an encoder 20 which canhave an elongated configuration, as shown in FIG. 1, or an annularconfiguration as shown in FIG. 2. In either case, encoder 20 hasmultiple magnetic pole pairs (MPP) 22 positioned at equally spacedintervals along the encoder. The encoder must made of metal or amagnetizable material. The preferred material is a synthetic materialwhich has been "loaded" with a highly magnetizable material, such asstrontium ferrite or barium ferrite. Other loading materials capable ofproviding higher magnetic flux densities per volume of material can beused, e.g., neodymium-iron-boron or samarium-cobalt, but strontiumferrite and barium ferrite will normally provide a sufficient magneticflux density at less expense and are therefore preferred from acost-efficiency point of view. The "loading factor" is the percentage,by volume, of magnetizable material loaded into an encoder. For example,a synthetic resin encoder with an 33% strontium ferrite loading factorwould consist of 33% strontium ferrite, by volume, and 67% syntheticresin, also by volume. The magnetic flux density will vary linearly andproportionately with the loading factor. For example, a strontiumferrite encoder with a loading factor of 33% would have a magnetic fluxdensity (F₃₃) which varies with the loading factor according to theequation:

    F=(K/0.33)×(F.sub.33)

F=Magnetic flux density corrected for loading factor K

F₃₃ =Magnetic flux density for 33% loading factor, and

K=Loading factor of the encoder

The magnetic flux is measured using a calibrated analog Hall-effectsensor. Specifically, the sensor is a UGS 3503 sensor manufactured bySprague packaged in a Sprague U package. This sensor has an activesensing area of 0.020"×0.020". The active sensing area is made up offour square Hall elements in a bridge configuration. All four elementscontribute to the sensor's output equally.

As the pole spacing of the magnet approaches the size of the active Hallelement size, an averaging of the magnetic field strength occurs. Thisaveraging effect causes the output from the sensor to be considerablyless than the magnetic field true strength.

An encoder with an 80% loading factor will have a magnetic flux desityabout twice that of a similar encoder with a 40% loading factor. Inaddition, if an encoder made of neodymium-iron-boron has a magnetic fluxdensity about 1.6 times as strong as that of a similar encoder made ofstrontium ferrite, both having the same loading factor, then aneodymium-iron-boron encoder with a 100% loading factor would have amagnetic flux density which is stronger than that of a strontium ferriteencoder, with an 33% loading factor, by a factor of 1.6/0.33 or about4.8.

The MPP can be oriented in various directions, depending on the productconfiguration. FIGS. 3 and 4 illustrate typical MPP orientations for aradial bearing encoder and a thrust bearing encoder, respectively. Themagnetic flux fields 33 are also depicted in phantom lines in FIGS. 3and 4. The peak flux 33 for a radial bearing encoder extends radiallyoutwardly from the encoder, as shown in FIG. 3. In contrast, the thrustbearing encoder of FIG. 4 has its maximum flux density emanating axiallyfrom the encoder. The most dense portion of the magnetic flux field isnormally oriented in the direction of a magnetic flux detector or sensorin order to optimize the sensitivity of the sensor for a specificproduct configuration. The orientation of the sensor itself will dependto some degree upon the type of sensor used; for example, a Hall sensormounted on an integrated circuit (I.C.) chip would be mountedperpendicularly relative to the surface of the encoder being sensed. Incontrast, a magnetoresistor sensor mounted on an I.C. chip would berotated 90 degrees from the orientation of a Hall sensor chip, in orderto provide optimal sensing of the encoder' s magnetic flux. As discussedabove, the flux density numbers provided herein have resulted from usinga Hall sensor whose sensitive portion has been positioned approximately0.036 inch from the encoder surface being measured.

The density of the magnetic flux must be sufficient to allow the sensorto detect the flux during the worst-case conditions, i.e., with amaximum air gap between the encoder and the sensor, at a maximumtemperature. The primary factor which can be controlled in order toprovide a desired threshold flux density is the loading factor, which isthe amount of magnetizable material, by volume, loaded into a syntheticbase material.

The method of magnetizing the encoder involves the same basic steps asdescribed above.

Various products and applications have different resolutionrequirements. In discussing the resolution of an encoder, the keyparameter is the number of magnetic pole pairs (MPP), which is closelyrelated to the pole spacing. FIG. 5 illustrates what is meant by polespacing by referring to the magnetic field produced by the MPP; inaddition, FIGS. 3 and 4 depict the pole spacing dimension with theletter "X". Pole spacing refers to the distance between adjacent MPP,e.g., from the peak flux density of a North pole to the peak fluxdensity of an adjacent South pole. Deciding which encoder configurationto use for a specific application entails many factors other than theresolution. For example, in a sensor bearing product application,consideration must be given to whether a radial bearing or a thrustbearing is necessary. In addition, the load to be carried by the bearingfactors into the type of bearing to be used, i.e., a ball bearing or aroller bearing, etc. Other factors would include whether or not to useroller elements at all, and if so, whether to use a cage or a retaineror a separator. Last, but not least, the final factor must be whetherthe optimal product configuration can utilize an encoder configurationwhich provides the resolution required. If not, another productconfiguration must be used.

In addition to the resolution requirement, another condition which mustbe met is the magnetic flux requirement. Even if the number of MPPrequired for a certain resolution can be provided, each MPP must alsoprovide a minimum or threshold magnetic flux density under worst-caseconditions in order to ensure that the sensor used will detect the fluxand produce a sufficient output signal as a result. For example, if thetemperature range within which a product must operate is known, thehighest possible temperature is used to calculate the magnetic fluxdensity, because the flux density decreases as the temperatureincreases, according to the following equation:

    F.sub.t =(F.sub.r){1-(K)(ΔT)}

where

F_(t) =Magnetic flux density corrected for actual temperature,

F_(r) =Magnetic flux density at reference temperature,

K=Thermal coefficient for a specific material, and

ΔT=Actual temperature-reference temperature.

The thermal coefficient for strontium ferrite and barium ferrite isabout 0.18% per degree Centigrade; therefore, the equation for strontiumor barium ferrite is:

    F.sub.t =(F.sub.r){1-(0.18% per degree C.)(ΔT)}.

Therefore, if the threshold flux density is provided at the highesttemperature which the product will endure, then the sensor will providean adequate output signal under the worst temperature conditions. Anyother factors, in addition to temperature, which would affect themagnetic flux density are similarly accounted for when determining thethreshold magnetic flux density.

Magnetizable materials are then considered in order to determine whichmaterial is optimal for providing the number of MPP and flux densitycombination derived in steps (a) through (c) above. Various rare earthmagnets, such as neodymium-iron-boron magnets, can provide especiallystrong magnetic flux densities with a minimal amount of material;however, their cost is higher than some weaker materials, such asstrontium ferrite and barium ferrite. The least expensive material whichmeets the resolution-flux requirements is normally chosen as the optimalmaterial.

For example, in many automotive applications an encoder made ofstrontium ferrite or barium ferrite would normally provide a sufficientnumber of MPP with adequate magnetic flux densitites; therefore, thesematerials would be used for these applications, instead of a strongerbut more expensive material, such as samarium-cobalt.

Once the material is selected, an encoder is manufactured by shaping thematerial to the desired encoder configuration and magnetizing thematerial with the number of MPP required to get the resolution desired.For example, an annular encoder, such as encoder 20 in FIGS. 2 through4, could have 360 MPP magnetized into a 2.6 inch diameter annular ringof strontium ferrite material. This encoder would be capable ofproviding a magnetic flux density of approximately 10 gauss, at adistance of approximately 0.036 inch from the surface of the magnet. Theencoder has a pole spacing of about 0.023 inch, and a thickness (T) of0.028 inch.

The thickness (T) of an encoder is an important variable because themagnetic flux generated by the encoder can be maximized for a fixedamount of encoder material by optimizing the ratio of the encoderthickness and the pole spacing. Research conducted by these inventorsindicates that a ratio of about 1.25 is optimal, i.e., the thickness ofthe encoder must be about 1.25 times the pole spacing dimension. A ratiorange of 1.25+/-25% has been found to be acceptable for many productapplications. FIGS. 3 and 4 clarify the thickness (T) and pole spacing(X) dimensions for annular encoders used in radial bearings and thrustbearings, respectively.

FIG. 6 graphs the optimal pole spacing-magnetic flux densityrelationship discussed above. For a required threshold magnetic fluxdensity for a known air gap, the largest usable pole spacing can bedetermined from the graph. This pole spacing will provide a specificnumber of MPP. For example, once the pole spacing for a given fluxdensity is determined, the number of MPP which can be magnetized into anannular encoder can be calculated using the following equation:

    Number of MPP=(D)(π)/X

where

D=Diameter of the annular encoder, and

X=Pole spacing.

A range of 80% to 100% of the number of MPP calculated for a particularencoder, for a given flux density, is considered to be an improvementover the existing prior art. The graph in FIG. 6 was developed usingsynthetic resin encoders made of strontium ferrite with an 33% loadingfactor. Flux density measurements were made at +20 degrees Centigrade,with an air gap of 0.036 inch between the outer surface of the encoderand the sensitive portion of a Hall sensor.

We claim:
 1. An encoder comprising a magnetizable member having athickness and a width, and multiple magnetic pole pairs (MPP) spaced atequal intervals along said member, said MPP having a pole spacingbetween adjacent pole pairs, said pole spacing having a range of valuesfrom about 0.01 inch to about 0.1 inch, each of said MPP having areference magnetic flux density (F_(r)) value in the range of about 2gauss to about 700 gauss at a distance of 0.036 inch from the encodersurface at a temperature of about +20 degrees Centigrade,each of saidF_(r) values being related to a range of pole spacing values by theequation:

    F.sub.r =(K.sub.1)(X)-(K.sub.2)(140X.sup.2)+(K.sub.3)(7920X.sup.3),

whereK₁ =158 to 1056 gauss per inch, K₂ =158 to 1056 gauss per inch², K₃=158 to 1056 gauss per inch³, X=Pole Spacing between adjacent MPP, andF_(r) =Reference magnetic flux density range (in gauss) at +20 degreesCentigrade, when the numerical values of K₁, K₂, and K₃ are equal.
 2. Anencoder according to claim 1, wherein said temperature varies within arange of about -40 degrees Centigrade to about +200 degrees Centigrade,and said reference magnetic flux density (F_(r)) varies with temperatureaccording to the following equation:

    F.sub.t =(F.sub.r){1-(K.sub.4)(ΔT)}

where F_(r) =reference magnetic flux density (in gauss) at 20 degreesCentigrade F_(t) =temperature-specific magnetic flux density ΔT=actualtemperature minus 20 degrees Centigrade; and K₄ =temperature coefficientfor said magnetizable member.
 3. An encoder according to claim 1,wherein said thickness of said member is optimized by making saidthickness equal to a factor of 1.25 times the pole spacing, whichcorresponds to a range of member thicknesses from about 0.0125 inch toabout 0.125 inch.
 4. An encoder according to claim 3, wherein saidthickness factor varies by +/-25%.
 5. An encoder comprising amagnetizable member made with a material from the group consisting ofstrontium ferrite and barium ferrite, with a loading factor of 33%, saidmember having a thickness and a width, and multiple magnetic pole pairs(MPP) spaced at equal intervals along said member, said MPP having apole spacing between adjacent pole pairs, said pole spacing having arange of values from about 0.01 inch to about 0.1 inch, each of said MPPhaving a reference magnetic flux density (F_(r)) value in the range ofabout 2 gauss to about 158 gauss at a distance of 0.036 inch from theencoder surface at a temperature of about +20 degrees Centrigrade,eachof said F_(r) values being related to a range of pole spacing values bythe equation:

    F.sub.r =(K.sub.5)(X)-(K.sub.6)(140X.sup.2)+(K.sub.7)(7920X.sup.3),

whereK₅ =158 to 230 gauss per inch, K₆ =158 to 230 gauss per inch², K₇=158 to 230 gauss per inch³, X=Pole Spacing between adjacent MPP, andF_(r) =Reference magnetic flux density range (in gauss) at +20 degreesCentigrade, when the numerical values of K₅, K₆, and K₇ are equal.
 6. Anencoder according to claim 5, wherein said temperature varies within arange of about -40 degrees Centrigrade to about +200 degrees Centigrade,and said reference magnetic flux density (F_(r)) varies with temperatureaccording to the following equation:

    F.sub.t =(F.sub.r){1-(K.sub.8)(ΔT)}

where F_(r) =Reference magnetic flux density at 20 degrees Centigradeand an 33% loading factor; F_(t) =Temperature-specific magnetic fluxdensity with an 33% loading factor; ΔT=Actual temperature minus 20degrees Centigrade; and K₈ =Temperature coefficient for strontiumferrite and barium ferrite (about 0.18% per degree Centigrade).
 7. Anencoder according to claim 6, wherein said temperature-specific magneticflux density (F_(t)) varies with said loading factor according to thefollowing equation:

    F=(K.sub.9 /0.33)×(F.sub.t)

where F=magnetic flux density for a specific temperature and loadingfactor, and K₉ =actual loading factor for strontium ferrite or bariumferrite.
 8. An encoder according to claim 5, wherein said thickness ofsaid member is optimized by making said thickness equal to a factor of1.25 times the pole spacing, which corresponds to a range of memberthicknesses from about 0.0125 inch to about 0.125 inch.
 9. An encoderaccording to claim 8, wherein said thickness factor varies by +/-25%.10. An encoder comprising an annular member made with a material fromthe group consisting of strontium ferrite and barium ferrite with aloading factor of 33%, having multiple magnetic pole pairs (MPP)circumferentially separated and circumferentially spaced around saidannular member and having a pole spacing of 0.01 inch, said encoderhaving an outer surface with a diameter in the range of about 0.5 inchto about 8 inches, and each of said MPP having a reference magnetic fluxdensity (F_(r)) in the range of about 2 gauss to about 158 gauss at adistance of 0.036 inch from said encoder surface at a temperature ofabout +20 degrees Centigrade,each of said flux density values beingrelated to a range of pole spacing values by the equation:

    F.sub.r =(K.sub.5)(X)-(K.sub.6)(140X.sup.2)+(K.sub.7)(7920X.sup.3)

whereK₅ =158 to 230 gauss per inch, K₆ =158 to 230 gauss per inch², K₇=158 to 230 gauss per inch³, X=Pole Spacing between adjacent MPP, andF_(r) =Reference magnetic flux density range (in gauss) at +20 degreesCentigrade, when the numerical values of K₅, K₆, and K₇ are equal. 11.An encoder according to claim 10, wherein said temperature varies withina range of about -40 degrees Centigrade to about +200 degreesCentigrade, and said reference magnetic flux density (F_(r)) varies withtemperature according to the following equation:

    F.sub.t =(F.sub.r){1-(K.sub.8)(ΔT)}

where F_(r) =reference magnetic flux density at 20 degrees Centigradeand an 33% loading factor; F_(t) =temperature-specific magnetic fluxdensity with an 33% loading factor; ΔT=actual temperature minus 20degrees Centigrade; and K₈ =temperature coefficient for strontiumferrite and barium ferrite=0.18% per degree Centigrade.
 12. An encoderaccording to claim 11, wherein said temperature-specific magnetic fluxdensity (F_(t)) varies with said loading factor according to thefollowing equation:

    F=(K.sub.9 /0.33)×(F.sub.t)

where F=magnetic flux density for a specific temperature and loadingfactor, and K₉ =actual loading factor for strontium ferrite or bariumferrite.
 13. An encoder according to claim 10, wherein the thickness ofsaid member is optimized by making said thickness equal to a factor of1.25 times the pole spacing, which corresponds to a thickness of about0.0125 inch.
 14. An encoder according to claim 13, wherein saidthickness factor varies by +/-25%.